<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        var age = window.prompt("请输入你的年龄");
        console.log(typeof age);
        // typeof可以判断变量类型
        if(age>=18){
            alert("可以进入网吧")

        }
        else{
            alert("在家呆着")
        }
    </script>
</body>
</html>